Abstract

Few studies investigating genital evolution examine the functional morphology of genitalia. In this study, we snap-froze copulating pairs of the millipede Antichiropus variabilis and used micro-computed tomography (CT) scanning and traditional light microscopy to investigate the mechanical interactions of male and female genitalia during copulation. The male genitalia of A. variabilis appear to function as lock-and-key structures, used in species mate recognition and species isolation. The female genitalia were complex in structure, and different sections of the male gonopods penetrated specific sections of the female genitalia. Morphological investigations confirmed a high degree of mechanical correspondence between male and female genitalia, as might be expected for a lock-and-key character. We discuss why genital shape is important in fertilization success for A. variabilis and present a novel application of micro-CT scanning that is widely applicable to other studies of genital evolution.